1. Field of the Invention
The present invention relates to a shielding method and shielding apparatus that shield leakage flux leaking from a magnetic circuit that generates alternating flux.
2. Description of the Related Art
One heating method is the IH (induction heating) method, whereby a body to be heated is induction-heated by a high-frequency magnetic field generated by causing a high-frequency current to flow in an exciting coil of a magnetic field generation circuit. A fixing apparatus that uses an induction heating apparatus employing this IH method as the heating means of the toner fixing section in an image forming apparatus is currently known (for example, Unexamined Japanese Patent Publication No.2001-5315).
The fixing apparatus described in Unexamined Japanese Patent Publication No.2001-5315 is configured, for example, so that the exciting coil of a magnetic field generation circuit is located in the vicinity of a fixing belt or fixing roller, a high-frequency current is caused to flow in this exciting coil, and the surface of the aforementioned fixing belt or fixing roller is induction-heated. Compared with a fixing apparatus in which the heating unit comprises a halogen lamp, a fixing apparatus that uses an induction heating apparatus employing this IH method as a heating unit has the advantages of higher thermal efficiency and lower energy loss, making possible rapid heating at low power consumption.
In an induction heating apparatus employing this IH method, there is generally a requirement for shielding of leakage flux leaking from the magnetic circuit that generates the alternating flux. As such a leakage flux shielding method, a method is currently known whereby a doughnut-shaped shielding ring of a conductive material such as aluminum is fitted around the exciting coil that is the source of leakage flux generation (for example, Examined Japanese Patent Publication No.SHO 58-37676).
In the method described in Examined Japanese Patent Publication No.SHO 58-37676, leakage flux is reduced based on the following principle. Namely, when an exciting coil provided in the magnetic field generation circuit of an induction heating apparatus is energized, a magnetic field is formed due to this energization, and an induction current is generated in the aforementioned shielding ring. Then, according to Lentz's law, when the flux linkage passing through the shielding ring changes, electromotive force arises in the direction preventing flux change, and an induction current flows in the shielding ring. The magnetic field generated from the shielding ring by this induction current is a magnetic field opposite in direction to the magnetic field generated from the exciting coil. Consequently, the magnetic field generated from the shielding ring and the magnetic field generated from the exciting coil cancel each other out, and generation of the aforementioned leakage flux is reduced.
The lower the resistance value of the shielding ring, the larger is the current that flows in the shielding ring, and the greater is the leakage flux reduction effect. However, making this shielding ring resistance value zero is difficult in practice. Therefore, with a conventional shielding method using this kind of shielding ring, it has not been possible to completely shield the above-described leakage flux.
Also, a characteristic of a high-frequency current is that, due to the skin effect, current does not flow deep within the conductor of the shielding ring, but mostly flows in the surface of the conductor. As a result, a problem with the above-described conventional shielding method is that, because of this skin effect phenomenon the induction current flowing in the shielding ring does not increase very much, and the leakage flux reduction effect does not improve, even if the thickness of the shielding ring is increased beyond a certain point.